1. Field of the Invention
This invention relates to devices used in drilling and boring through subterranean formations. More particularly, this invention relates to a polycrystalline diamond or other superabrasive cutting element intended to be installed on a drill bit, core bit or other tool used for earth or rock boring, such as may occur in the drilling or enlarging of an oil, gas, geothermal or other subterranean borehole, and to bits and tools so equipped.
2. State of the Art
There are three types of bits which are generally used to drill through subterranean formations. These bit types are: (a) percussion bits (also called impact bits); (b) rolling cone bits, including tri-cone bits; and (c) drag bits or fixed-cutter rotary bits, the majority of which currently employ diamond or other superabrasive cutting elements or "cutters," polycrystalline diamond compact (PDC) cutters being most prevalent.
In addition, there are other structures employed downhole, generically termed "tools" herein, which are employed to cut or enlarge a borehole or which may employ fixed superabrasive cutters on the surface thereof to engage the formation being penetrated. Such tools might include, merely by way of example, core bits, eccentric bits, bi-center bits, and reamers using both fixed and movable structures to carry the cutters. There are also fixed-cutter formation cutting tools employed in subterranean mining, such as drills and boring tools.
An exemplary drag bit or fixed-cutter bit is shown in FIG. 1. The drag bit of FIG. 1 is designed to be turned in a clockwise direction (looking downward at a bit being used in a hole, or counterclockwise if looking at the bit from its cutting end as shown in FIG. 1) about its longitudinal axis. The majority of current drag bit designs employ diamond cutters comprising polycrystalline diamond compacts (PDCs) mounted to, and in most cases actually formed on, a substrate, typically of cemented tungsten carbide (WC). State-of-the-art drag bits may achieve an rate of penetration (ROP) ranging from about one to in excess of one thousand feet per hour, depending on weight on bit (WOB), rotary speed, drilling fluid design and circulation rate, formation characteristics, and other factors known to those of ordinary skill in the art. A disadvantage of state-of-the-art PDC drag bits is that they may prematurely wear due to impact failure of the PDC cutters, as such cutters may be damaged very quickly if used in highly stressed or tougher formations composed of limestones, dolomites, anhydrites, cemented sandstones, interbedded formations such as shale with sequences of sandstone, limestone and dolomites, or formations containing hard "stringers." It is expected that the cutter of the invention will have use in the field of drag bits as a cutter on the face of a drag bit, and as a gage cutter or trimmer to maintain the gage, or diameter, of the borehole being drilled.
As noted above, there are additional categories of structures or "tools" employed in boreholes, which tools employ fixed superabrasive elements for cutting purposes, including core bits, eccentric bits, bi-center bits and reamers, the inventive cutter having utility in such downhole tools, as well as in fixed-cutter drilling and boring tools employed in subterranean mining.
It has been known in the art for many years that PDC cutters perform well on drag bits. A PDC cutter typically has a diamond layer or table formed, under ultra-high temperature and pressure conditions, onto a cemented carbide substrate (such as cemented tungsten carbide) containing a metal binder or catalyst such as cobalt. The substrate may be brazed or otherwise joined to an attachment member such as a stud or to a cylindrical backing element to enhance its affixation to the bit face. The cutting element may be mounted to a drill bit either by press-fitting or otherwise locking the stud into a receptacle on a steel-body drag bit, or by brazing the cutter substrate (with or without cylindrical backing) directly into a preformed pocket, socket or other receptacle on the face of a bit body, as on a matrix-type bit formed of WC particles cast in a solidified, usually copper-based, binder as known in the art.
A PDC is normally fabricated by placing a disk-shaped cemented carbide substrate into a container or cartridge with a layer of diamond crystals or grains loaded into the cartridge adjacent one face of the substrate. A number of such cartridges are typically loaded into an ultra-high pressure press. The substrates and adjacent diamond crystal layers are then compressed under ultra-high temperature and pressure conditions. The ultra-high pressure and temperature conditions cause the metal binder from the substrate body to become liquid and sweep from the region behind the substrate face next to the diamond layer through the diamond grains and act as a reactive liquid phase to promote a sintering of the diamond grains to form the polycrystalline diamond structure. As a result, the diamond grains become mutually bonded to form a diamond table over the substrate face, which diamond table is also bonded to the substrate face. The metal binder may remain in the diamond layer within the pores existing between the diamond grains or may be removed and optionally replaced by another material, as known in the art, to form a so-called thermally stable diamond ("TSD"). The binder is removed by leaching or the diamond table is formed in the first instance with silicon, a material having a coefficient of thermal expansion (CTE) similar to that of diamond. Variations of this general process exist in the art, but this detail is provided so that the reader will understand the concept of sintering a diamond layer onto a substrate in order to form a PDC cutter. For more background information concerning processes used to form polycrystalline diamond cutters, the reader is directed to U.S. Pat. No. 3,745,623, issued on Jul. 17, 1973, in the name of Wentorf, Jr. et al.
Prior art PDCs experience durability problems in high load applications. They have an undesirable tendency to crack, spall and break when exposed to hard, tough or highly stressed geologic structures. The durability problems of prior art PDCs are worsened by the dynamic nature of both normal and torsional loading during the drilling process, wherein the bit face moves into and out of contact with the uncut formation material forming the bottom of the wellbore, the loading being further aggravated in some bit designs and in some formations by lateral vibration of the bit and drill string or by so-called bit "whirl."
The diamond table/substrate interface of conventional PDCs is subject to high residual stresses arising from formation of the cutting element, as during cooling, the differing coefficients of thermal expansion of the diamond and substrate material result in thermally-induced stresses. In addition, finite element analysis (FEA) has demonstrated that high tensile stresses exist in a localized region in the outer cylindrical substrate surface and internally in the substrate. Both of these phenomena are deleterious to the life of the cutting element during drilling operations as the stresses, when augmented by stresses attributable to the loading of the cutting element by the formation, may cause spalling, fracture or even delamination of the diamond table from the substrate.
Further, high tangential loading of the cutting edge of the cutting element results in bending stresses on the diamond table, which is relatively weak in tension and will thus fracture easily if not adequately supported against bending. The metal carbide substrate on which the diamond table is formed is typically of inadequate stiffness to provide a desirable degree of such support.
The relatively thin diamond table of a conventional PDC cutter, in combination with the substrate, also provide lower than optimum heat transfer from the cutting edge of the cutting face, and external cooling of the diamond table as by directed drilling fluid flow from nozzles on the bit face is only partially effective in reducing the potential for heat-induced damage.
The relatively rapid wear of conventional, thin diamond tables of PDC cutters also results in rapid formation of a wear flat in the substrate backing the cutting edge, the wear flat reducing the per-unit area loading on the rock, reducing stress thereon in the vicinity of the cutting edge and thus requiring greater weight on bit (WOB) to force the cutters into the rock and maintain rate of penetration (ROP). The wear flat, due to the introduction of the substrate material as a contact surface with the formation, also increases drag or frictional contact between the cutter and the formation due to modification of the coefficient of friction. As one result, frictional heat generation is increased, elevating temperatures in the cutter, while at the same time the presence of the wear flat reduces the opportunity for access by drilling fluid to the immediate rear of the cutting edge of the diamond table.
Others have previously attempted to enhance the durability of conventional PDC cutters. By way of example, the reader is directed to U.S. Patent Re. 32,036 to Dennis (the '036 patent); U.S. Pat. No. 4,592,433 to Dennis (the '433 patent); and U.S. Pat. No. 5,120,327 to Dennis (the '327 patent). In FIG. 5A of the '036 patent, a cutter with a beveled peripheral edge is depicted, and briefly discussed at col. 3, lines 51-54. In FIG. 4 of the '433 patent, a very minor beveling of the peripheral edge of the cutter substrate or blank having grooves of diamond therein is shown (see col. 5, lines 1-2 of the patent for a brief discussion of the bevel). Similarly, in FIGS. 1-6 of the '327 patent, a minor peripheral bevel is shown (see col. 5, lines 40-42 for a brief discussion of the bevel). Such bevels or chamfers were originally designed to protect the cutting edge of the PDC while a stud carrying the cutting element was pressed into a pocket in the bit face. However, it was subsequently recognized that the bevel or chamfer protected the cutting edge from load-induced stress concentrations by providing a small load-bearing area which lowers unit stress during the initial stages of drilling. The cutter loading may otherwise cause chipping or spalling of the diamond layer at an unchamfered cutting edge shortly after a cutter is put into service and before the cutter naturally abrades to a flat surface or "wear flat" at the cutting edge.
It is also known in the art to radius, rather than chamfer, a cutting edge of a PDC cutter, as disclosed in U.S. Pat. No. 5,016,718 to Tandberg. Such radiusing has been demonstrated to provide a load-bearing area similar to that of a small peripheral chamfer on the cutting face.
U.S. Pat. No. 5,351,772 to Smith discloses a PDC cutter having a plurality of internal radial lands to interrupt and redistribute the stress fields at and adjacent the diamond table/substrate interface and provide additional surface area for diamond table/substrate bonding, permitting and promoting the use of a thicker diamond table useful for cutting highly abrasive formations.
U.S. Pat. No. 5,435,403 to Tibbitts discloses a PDC cutter employing a bar-type, laterally-extending stiffening structure adjacent the diamond table to reinforce the table against bending stresses.
For other approaches to enhance cutter wear and durability characteristics, the reader is also referred to U.S. Pat. No. 5,437,343, issued on Aug. 1, 1995, in the name of Cooley et al. (the '343 patent); and U.S. Pat. No. 5,460,233, issued on Oct. 24, 1995, in the name of Meany et al. (the '233 patent). In FIGS. 3 and 5 of the '343 patent, it can be seen that multiple, adjacent chamfers are formed at the periphery of the diamond layer (see col. 4, lines 31-68 and cols. 5-6 in their entirety). In FIG. 2 of the '233 patent, it can be seen that the tungsten carbide substrate backing the superabrasive table is tapered at about 10.degree.-15.degree. to its longitudinal axis to provide some additional support against catastrophic failure of the diamond layer (see col. 5, lines 2-67 and col. 6, lines 1-21 of the '233 patent). See also U.S. Pat. No. 5,443,565, issued on Aug. 22, 1995, in the name of Strange for another disclosure of a multi-chamfered diamond table.
While the foregoing patents achieved some enhancement of cutter durability, there remained a great deal of room for improvement, particularly when it is desired to fabricate a cutter having, as desirable features, a relatively larger and robust diamond volume offering reduced cutter wear characteristics and increased stiffness. Conventional PDCs employ a diamond table on the order of about 0.030 inch thickness. So-called "double-thick" or 0.060 inch thick diamond tables have been attempted, but without great success due to low strength and wear resistance precipitated to some degree by poorly-sintered diamond tables. It has even been proposed to fabricate PDC cutters with still-thicker chamfered diamond tables, as thick as 0.118 inch, as disclosed in U.S. Pat. No. 4,792,001 to Zijsling. However, the inventor is not aware of the actual manufacture of any such cutters as disclosed by Zijsling.
Yet another cutter bearing an extremely thick diamond table known to the inventor has been developed, such cutter comprising a PDC or other compact of other superabrasive table of substantially enhanced thickness and durability. The aforementioned cutter is disclosed and claimed in co-pending U.S. patent application Ser. No. 08/602,076, now U.S. Pat. No. 5,706,906 filed Feb. 15, 1996 and assigned to Baker Hughes Incorporated, the assignee of the present invention. An exemplary embodiment of the cutter of the '076 application (hereinafter the "'076 cutter") is depicted in FIGS. 2a through 2d of the drawings. The reader is referred to the aforementioned '076 application for a more detailed physical description of the '076 cutter, variations thereof and their characteristics, but some significant aspects of the '076 cutter as regards the present invention are hereinafter set forth.
Reference is made to FIGS. 2a through 2d which depict an end view, a side view, an enlarged side view and a perspective view, respectively, of an exemplary embodiment of the '076 cutter. The cutter 501 is of a shallow frustoconical configuration and includes a circular diamond layer or table 502 (e.g. polycrystalline diamond) bonded (i.e. sintered) to a cylindrical substrate 503 (e.g. tungsten carbide). The diamond layer 502 is of a thickness "T.sub.1." The substrate 503 has a thickness "T.sub.2." The diamond layer 502 includes rake land 508 with a rake land angle .theta. relative to the side wall 506 of the diamond layer 502 (parallel to the longitudinal axis or center line 507 of the cutter 501) and extending forwardly and radially inwardly toward the longitudinal axis 507. The rake land angle .theta. is defined as the included acute angle between the surface of rake land 508 and the side wall 506 of the diamond layer which, in a cylindrical cutter as shown, is parallel to longitudinal axis 507. The rake land itself is preferably about 0.050 inch wide, measured radially along the surface of the rake land (W.sub.1).
Diamond layer 502 also includes a cutting face 513 having a flat central area 511 radially inward of rake land 508, and a cutting edge 509. Between the cutting edge 509 and the substrate 503 resides a portion or depth of the diamond layer referred to as the base layer 510, while the portion or depth between the flat central area 511 of cutting face 513 and the base layer 510 is referred to as the rake land layer 512. The central area 511 of cutting face 513, as depicted in FIGS. 2a, 2b, 2c and 2d, is a flat surface oriented perpendicular to longitudinal axis 507.
In the depicted cutter, the thickness T.sub.1 of the diamond layer 502 is in the range of 0.070 to 0.150 inch, with a most preferred range of 0.080 to 0.100 inch. The rake angle .theta. of the rake land 508 as shown is 65.degree. but may, as previously noted, vary. The boundary 515 of the diamond layer and substrate to the rear of the cutting edge lies at least about 0.015 inch longitudinally to the rear of the cutting edge (T.sub.3).
An optional cutter feature proposed for the '076 cutter and depicted in broken lines in FIG. 2a is the use of a backing cylinder 516 face-bonded to the back of substrate 503.
The '076 cutter has demonstrated, for a given depth of cut and formation material being cut, a substantially enhanced useful life in comparison to prior art PDC cutters due to a greatly reduced tendency to catastrophically spall, chip, crack and break. It has been found that the cutter in PDC form may tend to show some cracks after use, but the small cracks surprisingly do not develop into a catastrophic failure of the diamond table as typically occurs in prior art PDC cutters. This capability, if fully realized, would be particularly useful in a cutter installed on a drag bit to be used on hard rock formations and softer formations with hard rock stringers therein (mixed interbedded formations) which are currently not economically drillable with PDC cutters.
While the '076 cutters with their large rake lands have shown some promise in initial field testing, conclusively proving the durability of the design when compared to other cutters of similar diamond table thickness but without the large rake land, the '076 cutters have also demonstrated some disadvantageous characteristics which impair their usefulness in real-world drilling situations. Specifically, drill bits equipped with the '076 cutters demonstrate a disconcerting tendency, apparently due to the extraordinarily great cutting forces generated by contact of these cutters with a formation being drilled, to overload drilling motors, other bottomhole assembly (BHA) components such as subs and housings, as well as tubular components of the drill string above the BHA.
Further, bits equipped with the '076 cutters often drill significantly slower, that is to say, their rate of penetration (ROP) of the formation is far less than bits equipped with conventional cutters, and also exhibit difficulty in drilling through hard formations for which they would be otherwise ideally suited. It appears that the exterior configuration of these thick diamond table cutters, although contributing to the robust nature of the cutters, may be less than ideal for many drilling situations due to the variable geometry of the arcuate rake land as it contacts the formation and attendant lack of "aggressiveness" in contacting and cutting the formation. It is conceivable, as demonstrated in the cutting of metal with similarly-shaped structures, that in plastic formations the '076 cutter may simply deform the material of the formation face engaged by the cutter, forming a plastic "prow" of rock ahead and flanking the cutter, instead of shearing the material as intended.
Therefore, despite the favorable characteristics exhibited by the '076 cutter, its utility in efficiently cutting the difficult formations for which its demonstrated durability is ideally suited remains, as a practical matter, unrealized over a broad range of formations and drilling conditions. Further, and as noted with regard to the other cutter designs discussed above, there remains a need for a robust superabrasive cutter which will withstand cutting stresses in the difficult formations referenced above, while drilling effectively with, and without damage to, conventional, state-of-the-art bottomhole assemblies and drill strings, and providing commercially viable, consistent ROP.